Cracking hydrocarbon oil



May 30, 1939.

S. TIJMSTRAV CRACKvING HYDROCARBON OIL Filed March 20, 1956 2 Shoae'ts-Shee'l'I l are s. TIJMs'rRA CRACKING HYDROCARBON CIL 'May 3Q, 1939.

195e 2 sheets-sheet 2 Filed March 20,

EE coicom xcE. :Encan G uvam Som -Bg his Ajriorne Patented May 30, 1939' AUNITED STATES CRACKIN G HYDROCARBON OIL Sli-bren ,Tijmen-a, Berkeley, Calif., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware application March zo, 193s, serial No. 69,791 3 clams. (ci. 19e-5o) 'lhis invention relates to a process of cracking hydrocarbon oils,consisting in the conversion of vheavier hydrocarbons into lighter hydrocarbons. usually of gasoline or similar boiling ranges, the

5 conversion being eiected by heat, and usually under superatmospheric pressure.Y Particularly, my'invention relates to improvements in such processes whereby the formation of coke and similar objectionable carbonaceous materials in the cracking coils and other parts of the apparatus is to the large extent prevented.

Cracking operations, as usually carried out, are attended with the formation of large amounts of undesirable carbonaceous materials, such as coke, tar, and other highly polymerized hydrocarbons, along with the gasoline and similar fractions. Besides constituting a waste of hydrocarbons, the formation of these materials is objectionable because it causes the overheating of the 20 cracking coils; it has heretofore been attempted to reduce the formation of such substances by a judicious choice of operating conditions, or

by subjecting the initial hydrocarbon mixture to a preliminary refining treatment, as by removing 25, asphaltic materials from the stock charged to the cracking coils.

I have found that the removal of asphaltic substances, such as asphaltenes, from charging or recycle stock in the usual way, as by precipi tation ofY solid matter with propane or similar hydrocarbons, does not effectively reduce the formation of the objectionable carbonaceous materials, but that a material improvement is achieved by subjecting the hydrocarbon oil to be fed" to the cracking coils to the precipitating action of low molecular hydrocarbons, particularly aliphatic hydrocarbons of less than seven carbon `atoms in the moleculeand, preferably, the so-called permanent gases of less than four carbon atoms, under a high pressure, and -at a temperature near to or above the critical temperature of the gases, whereby the oil is caused to separate into two liquid phases of diierent specific gravities. One particular form of the treatment consists of producing separable' nongaseous phases by mixing the cracked material with a gaseous treating agent under temperature and pressure conditions which are less intense than its critical condition but which are insufiicient to cause the condensation of the treating agent. The heavier liquid phase (which may often be extremely viscous, or semi-solid) is separated from the lighter liquid phase, and the latter is utilized as the charging s tock to the cracking coil, or as a source for the cracking the present invention is especially useful inconstock. 'I'he pressure of the treatment is normally between about 20 and 200 atmospheres pressure, therebeing suiiicient low-boiling gas in the treating vessel 'to cause the normally liquid hydrocarbons to be substantially saturated with the 5 v gaseous hydrocarbonsny'lhis treatment may be applied to the cracked residue withdrawn from the cracking furnace, or to the initial feed oil, or to both, separately or in admixture. While 10 nection with liquid pha'secracking processes, it may also be applied to the treatment of cracked products resulting from a vapor phase cracking process.

A My invention will be described with reference to the accompanying drawings, in which Figures 1 and 2 are schematic iiow diagrams illustrating schematically the arrangement of the apparatus suitable for carrying out the invention, it being understood that theinvention may be operated in many other types of apparatus, and under conditions other than those particularly described, without departing from the spiritand scope of the invention. -While a simple forrnl of cracking unit has been illustrated,l any other form may be substituted therefor; and since the operation of such units is well understoodin the art, the following specication will describe in detail only the particular improvements conl-stituting the present invention.

Referring to Figure 1, I represents a furnace provided with suitable heating coils and burners. A chargingistock, consisting qf uncracked stock and recycle stock, is fed to the furnace I through a conduit 2, although the uncracked stock and recycle stocks may in certain cases be cracked separately. The cracked products may be withdrawn from the furnace at 3, and fed directly to a transfer line 4 through a valve I. If desired, the valve 5 may be closed, and the valve 6 opened, thereby 'flowing the cracked materials through a reaction zone 1, from-'which cracked stock may be withdrawn at 8. 'Ihe rate of re action may be controlled by introducing a cooling medium, such as a cooled gas obtained in i the stabilizer unit, or a portion of the raw feed from the tank 9, through a conduit I0; or byv quenchingv the material in the line l by introducing the cooling medium through a valve Il.

The cracked materials, instead of being fed into vthe usual vaporizingunit, in which the pressure is usually reduced andthe vaporimble material separated from, the cracked residue, are, according to my process, treated with a low molecular gaseous treating agent at a high pressure u to effect a diierent type of separation. The cracked products may be treatedA either alone, or together with the raw feed. According to one mode of treatment, the mixture is fed through a throttle valve I 5 to a multiple outlet valve I6, constructed to divert the iiow of cracked material to one of several settling tanks I1, and I8, it being understood that any number of these tanks may be provided. If desired, the cracked products may iiow to an indirect heat exchanger to,

` the mixture boiling above about the boiling temperature of aliphatic hydrocarbons having between about three to six carbon atoms may be liquid, although it is also possible to select temperatures and pressures at whiclithe greater portion of the gasoline hydrocarbons are vaporized, and only higher boiling hydrocarbons are in the liquid state.

The gas is introduced until the proper pressure is established, and a sufiicient quantity of gas at a para-critical temperature is dissolved in the liquid to cause the latter to separate into two liquid phases. 'Ihe separation of the liquid material into two liquid phases is occasioned by the lowering of the density of the liquid oil in the tank I1 caused by the introduction of the gas, and it should be noted that enough space must be left in the tank I1 prior to the introduction of the gas to allow room for this expansion.

While I have shown the addition of low boiling hydrocarbons to the tank I1, it should be noted that the cracked materials in the transfer line l.

and .effective to cause the separation of the heavy hydrocarbon mixture into two liquid phases. In this case it is only necessary to increase the pressure on the system to vcause these gases to be dissolved in the liquid oil, thereby lowering the density of the latter and causing a separatio into two liquid phases. s

The temperature in the settling tank of phase separator I1 must be high enough to cause the low-boiling hydrocarbons which are active in causing the formation of the liquid phases to be in the para-critical state. A substance is said to be in its para-critical state when it is above, at, or slightly below its critical temperature, the lower limit being about 40' C. below the critical temperature. Since the mixture in the transfer line I contains a great diversity of hydrocarbons of varying molecular sizes and critical temperatures, it will be noted that when the temperature in the chamber I1 is comparatively low, fewer of these hydrocarbons will be in the para-critical state than if higher temperatures are employed, and it becomes necessary to add more low-boiling hydrocarbons from the tank Il, in order to have l' would, for practical reasons,` rarely be employed.

The higher boiling hydrocarbons, such as pentane and hexane, are less desirable and with many types of hydrocarbon oils it is diillcult to cause the formation of two liquid phases with these hydrocarbons because, when usedA alone, they necessitate the use of high temperatures. generally above 200 C., and because such high temperatures promote miscibility, thereby preventing the formation of two liquid phases. The presence of such hydrocarbons which have slightly higher molecular weights than the hydrocarbons in the para-critical state and which are present in the system in the liquid state under the conditions of the treatment is, however, benecial, since they j promote the solubility of the active hydrocarbons, and aid in lowering the density of the mixture being treated.

'I'he temperature should always be wellbelow the critical temperature of the composite mixture introduced into the separator I1 and, in general, I prefer to operate the phase separator at a temperature between about 20 C. and 250 C., and at a pressure between about 20 and 200 atmospheresl When raw stock has been introduced through the line I0, the resulting mixture of gas and stock fed into the tank I1 will often have the desired temperature; if not. the mixture may be cooled by introducing raw stock into the tank I1 through a valve 2I, or by flowing v the raw stock through a valve 22 and cooling coil 2l. Alternately, the mixture may be passed through a heat-exchanger before introduction into the tank I1.

- As a result of the formation of two liquid phases by the above described method, the'mixture will be separated into two portions: most of the lowboiling hydrocarbonsl which are in the paracritical state. and other low-boiling hydrocarbons, such as gasoline, and the like, as well as heavier hydrocarbons which are suitable i'or use as charging stock to the cracking unit, are concentrated in the lighter liquid phase, or are present in a gas phase above the liquid phase; and the heavier phase will consist mainly of heavy hydrocarbons which are largely responsible for the formation of coke in the cracking unit.

'I'he lighter and heavier liquid phases from the separator I1 are separately withdrawn, by withdrawingthe liquid material through valves 2l and 25, the latter valve being set to direct the flow tothe conduit 26 until all of the heavier phase has been withdrawn. whereupon the valve 2B is set to divert the ilow of the lighter phase to the conduit 21. The light and heavy phases are then fed to the expanding vessels 2| and 2l. respectively, where the permanent gases or other highly yolatile vgases are separated under a re-vv duced pressure, the gases being conducted to the tank Il through conduits ll and lI. rel.

` turned to the tank I8 through the low pressure line 32 by opening the valve If, however. the tank I1 is operated under a pressure low enough `tovapcrize a substantial portion of gasoline and other desirable hydrocarbons. these may be represence of a gas phase'is, of course, not essential,

covered by flowing the gas. through a valve 8l to the stabilizer unit, subsequently described.

After the tank II has been emptied, the ow of cracked material from the still is again diverted to the tank II, and the material accumulated in the tank I8 is treated in the manner described above for the tank Il, corresponding apparatus being indicated by primed numerals.

The liquid hydrocarbons from the expansion vessel 28 are fractionated in the fractionating column 3l, which may be operated at any suitable temperature and pressure to produce a bottom product at 88 containing hydrocarbons of boiling temperature ranges suitable for' use as the cracking stock, and an overhead product at Il, containing the lower boiling liquid hydrocarbons and gaseous hydrocarbons which were not separated in the expansion vessel 28. All or a portion of the overhead product from 8l is condensed in a condenser 98, and recycled as reiiux, the portion which is not recycled being further treated in a stabilizer column 89, for the separation. of additional quantities of gases, which may be introduced into the tank I9 through a conduit ll or withdrawn at II. The stabilized -bottom product from the column 89 may be withdrawn at l2 and stored in tank It 'I'he fractionating column may be operated in any desired manner, as is well known in the art. Heat may be supplied to this column by withdrawing light gases from the reaction chamber 'I through a conduit 48; or incondensablev or other light hydrocarbons may be sent through the cooling coils 28, 28' instead of the raw feed, and the heatedhydrocarbons may be fed to the bottom' of the tower 8l as shown in Figure 2. The raw feed may, if desired, be introduced into the column 95 through a conduit III, either after passing through heat exchangers 28, 28', or through the valve I.

The bottoms from the expansion vessel 29, withdrawn at u, normally consist of highly viscous aromatic'oils often containing large amounts of asphaltic and resinous materials, and may be disposed of in any desired manner. When the lower phase inthe separator I'I contains appreciable quantities of hydrocarbons of gasoline and similar boiling temperature ranges, it may be desirable to fractionate the bottoms from the vessel 29.'in a steaming column l5, and return the more'volatile constituents to the fractionating column 85, or to the stabilizer 39 through a conduit 46, the `viscous aromatic and other undesirable materials being withdrawn at I'I.

'I'he vessels 28 and 29, and .the stabilizer unit 39 are operated to accumulate in the tank I9 only gases having molecular weights less than 80, and preferably less than 50. 'Ihese hydrocarbons may beof the saturated parafnic orof the oleflnic type. If desired, these gases may be desulfurized in the desulfurizing apparatus I2 by closing the valve I3 and opening the valve Il. This treatment may, for example, comprise the step of washing the gas with a solution of potassium phosphate, or some other alkaline absorbing agent, capable of removing hydrogen sulfide. the gas in this treatment. The quantity of the gaseous hydrocarbons supplied from the tank I9 to the tank I1 or I8 depends upon the quantity of similar gases which are present in the crackedv equilibrium with the two'liquid phases. The

Mercaptans may also be removed from but is preferred f or the purpose of insuring'that a sumcient quantity of gas is present in the system to saturate the liquid hydrocarbons at the prevailing. pressure. It should, moreover, be noted that while I prefer to saturate the liquid hydrocarbons with the gas. this is not always necessary, since the formation Aof two liquid phases is possible even though lesser amounts of gas are employed. This is true particularly when higher temperatures, in the vicinity of o-250 C. are employed.

As a modification of the above process. I may I introduce all or part of the low-boiling hydrocarbons from the tank I9 through the valve 28' and digest the cracked oil in the reactor 1 in the presence of the added low-boiling hydrocarbons.

This is often eilective to prevent the clogging of gnduitlnes between the furnace and the tanks and It is also possible to operate the process with` .sure in the cracking coils may at times be sumcient for this purpose, but in most processes it is necessary to increase the pressure by means of a pump.

As was indicated above, the raw feed stock may be similarly subjected tothe treatment described above in connection with thetanks I'I and I8.. in the absence of the cracked materials. 'I'he treated feed stock may then be fed either directly to the cracking coils or into the fractionating column 85.

While I have in the foregoing specication a vparticularly described the use of gaseous hydrocarbons for causing the formation of the nongaseous phases in the tanks I'I and I8, because these gases are most conveniently available in cracking plants 'it should be noted that other low-molecular gaseous' substances having low critical temperatures, preferably below 250 C. may also be employed. Examples of sucll gaseous treating agents are: sulfur dioxide, carbon dioxide, carbon monoxide, methyl iluoride, hydrogen,` etc. Figure 2 illustrates a modified ow diagram the process, similar reference characters designating the same elements as in Figure 1. According to this modified arrangement, .the incondensable gases from the tank I9 are passed through the coils 23 and 23 to regulate the temperature` of the cracked products in the tanks I'I and I8. The heated gases are then fed into the bottom 0f the fractionating column 35 via conduits I0 and I8, either in addition to or in lieu vof the may be introduced either directly into the cracking furnace via conduit III as shown in Figure 2, or it may be passed through the settling tanks or the fractionator. l

I claim as my invention:

1. In a process for converting heavy hydrocarbons into lighter oils, comprising-the steps of A subjecting the heavy oil to pyrogenous cracking conditions, subjecting the cracked oilv containing a substantial portion of the gasoline components* and heavier componentsfin a separating zone to the precipitating action of a quantity of low molecular hydrocarbons having less than four carbon atoms at a separating temperature at which said low molecular hydrocarbon is in the para-critical state, at a pressure at which said.

gasoline components and heavier components are 'reduced to the liquid state and at which a sufflcient quantity of said low molecular hydrocarbons are dissolved in the liquid cracked oil to cause the formation of two liquid phases having diierent specic gravities, separating said' liquid phases, iractionating hydrocarbons contained in the separated lighter phase in a fractionating zone, the improvement consisting of heat-interchanging the cracked hot oil with an'incondensable hydrocarbon gas in said separating zone, thereby heating the gas 'and cooling the oil to the separating temperature. passing the heated gas into said fractionating zone as a heating medium therefor, separating a heavy hydrocarbon fraction from vthe fractionating zone. and subjecting said heavy fraction to cracking.

2. The process according to claim 1 in which l0 the separating' temperature is above 150 C.

3. The process according to claim 1 in which the separating temperature is above the critical temperature of the low molecular hydrocarbon.

SIJBREN TIJMSTRA. 

